1. Set your strap lengths so your cameras don't bang into each other.
2. Medium to long zoom lenses are best carried "backwards".
It's a simple thing, but very important...if you're carrying two bodies make sure to adjust the straps to different lengths so that your cameras do not bang together while you are walking. I carry my wide zoom on my left shoulder and my long zoom on my right shoulder. The strap on my wide 16-35 is set shorter and the strap on my 70-200 lens is set longer. I'm right handed, so to get my short lens, I reach across my body, to get my telephoto, I just reach down. If you are left handed you might prefer to reverse it.
Haiti 2003 Photo copyright Mike Thorne
Some folks carry their long zoom lenses with the camera back touching their body....the heavy weight of the lens pulls down and the camera and it ends up "off balance" . If you reverse the lens, it tucks nicely against your body with both the lens and the camera body touching you. Less is swinging out in the breeze and you are less likely to bang it into something. Also, when walking through crowds you are "narrower".
The short zoom doesn't matter as much since it's not so "lens heavy" or "front heavy" and therefore it doesn't' pivot downwards. The balance of a Nikon D1X or D2X seems better than the Mark II, which is a function of how the straps attach to the camera. The D1X hung nicely with the viewing screen flat against me, whereas the Mark II tends to pivot down a bit, making it less comfortable...oh well.
Canon straps (and Nikon) straps come with a high friction backing on the straps which helps keep them from sliding off your shoulder, which is fairly effective. I find that wearing a shirt with a seam down the top of the shoulder is an additional help to keep the straps from sliding down. When you start sweating heavily your straps and your shirt get "slippery" so every little bit helps.
Straps do wear out, so you need to be vigilant about checking them for signs of wear. For more on this topic click here for my thoughts on camera strap replacement
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